The invention concerns a carbon film, consisting of a colorless plastic film as base, a coloring carbon layer and a thin intervening layer located between the base and the carbon layer.
Carbon films of this type serve in sheet form for the production of copies when an original is typed in a typewriter, or as typewriter ribbons. In the sense of the invention the term "carbon layer" is to be understood in the broadest sense and is to include all those layers which contain lampblack or other pigments or e.g. magnetic pigments, insofar as the latter possess a sufficiently dark color.
In principle it is desirable to prepare the back of the finished product for accepting graphics, texts and graphic and verbal representations of different types. This presupposes an appropriate coloring of the back of the film, which cannot be attained at all simply, since the coloring side of the carbon paper is deep black.
In the past, paper was used as the base of the carbon layer in the production of copies, and in order to design the back of the paper, a covering layer with a desired color was applied, whereby it was next necessary to print the desired graphic and verbal representations on the covering layer in an additional operation. Of course, in the use of paper as a base for the carbon layer, an additional coating of the back was necessary anyway, because only in this way could the paper, having been provided with the carbon layer, be prevented from rolling because of unbalanced tensions in the coating, thus making practical use more difficult.
Recently paper has been replaced as the base by transparent plastic film, in the case of which the danger of rolling up does not arise, so that, as far as this is concerned, an additional coating of the back is no longer necessary. This raises the possibility of using the back of the film directly as an advertising medium of special value, if it can be successfully finished, not only graphically but also in color.
In the use of plastic film it is necessary and usual to apply an intervening layer as well to improve the adhesion of the carbon layer. This intervening layer, which is usually prepared to be antistatic as well, has a thickness of only about 2 to 10 microns. Because of this slight thickness of the intervening layer, however, one cannot avoid having the carbon material impinge on this layer or strike through it, whereby the back of the film presents an unattractive appearance, being largely dull black. The same effect arises when the intervening layer is pigmented with usual pigments, i.e. even then no color effect appears on the back of the film. For this reason, up until now a covering layer has been applied to the back of the film for optical reasons, even with carbon film, in which case it was also necessary to apply any lettering afterward in an additional operation, after the application of the covering layer.